_ _  |  |  _ _



 _ ______________


Terminate Program (Function 00H)

 Call:

 AH = 00H
 CS
  Segment address of
  Program Segment Prefix

 Return:

 None



Comments:

Function 00H performs the same function as Interrupt 20H. It terminates
the current process and returns control to its parent process. It also closes
all open file handles and clears the disk cache. When this interrupt is
issued, CS must contain the segment address of the Program Segment
Prefix.

The CS register must contain the segment address of the Program Seg-
ment Prefix before you call this interrupt.

The following exit addresses are restored from the specified offsets in the
Program Segment Prefix:

Offset
 Exit Address
_ ________________________________________________________________

0AH Program terminate

0EH CONTROL-C

12H Critical error

All file buffers are flushed to disk.


  1

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

_ ________________________________________________________________

Warning 

 Close all files that have changed in length before calling this function.
 If you do not close a changed file, its length is not correctly recorded in
 the directory. See Function 10H for a description of the Close File sys-
 tem call.

_ ________________________________________________________________


Macro Definition:


terminate_program macro
   xor ah,ah
   int 21H
   endm


Example:

The following program displays a message and returns to MS-DOS. It uses
only the opening portion of the sample program skeleton shown in Figure
1.2.

message db "Displayed by FUNC00H example", 0DH,0AH,"$"
;
begin: display message ;see Function 09H
 terminate_program ;THIS FUNCTION
code ends
 end start



2

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Read Keyboard and Echo (Function 01H)

 Call:

 AH = 01H

 Return:

 AL
  Character typed



Comments:

Function 01H waits for a character to be read from standard input, then
echoes the character to standard output and returns it in AL. If the char-
acter is CONTROL-C, it executes Interrupt 23H.

Macro Definition:


read_kbd_and_echo macro
   mov ah, 01H
   int 21H
   endm


Example:

The following program displays and prints characters as you type them. If
you press the RETURN key, the program sends a linefeed/carriage-return
sequence to both the display and the printer.

begin: read_kbd_and_echo ;THIS FUNCTION
 print_char al ;see Function 05H
 cmp al,0DH ;is it a CR?
 jne begin ;no, print it
 print_char 0AH ;see Function 05H
 display_char 0AH ;see Function 02H

  3

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

 jmp begin ;get another character



4

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Display Character (Function 02H)

 Call:

 AH = 02H
 DL
  Character to be displayed

 Return:

 None



Comments:

Function 02H sends the character in DL to standard output. If you press
CONTROL-C, it issues Interrupt 23H.

Macro Definition:


display_char macro character
  mov dl,character
  mov ah,02H
  int 21H
  endm


Example:

The following program converts lowercase characters to uppercase before
displaying them.

begin: read_kbd ;see Function 08H
 cmp al,"a"
 jl uppercase ;don't convert
 cmp al,"z"
 jg uppercase ;don't convert
 sub al,20H ;convert to ASCII code
 ;for uppercase

  5

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

uppercase: display_char al ;THIS FUNCTION
 jmp begin: ;get another character



6

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Auxiliary Input (Function 03H)

 Call:

 AH = 03H

 Return:

 AL
  Character from auxiliary device



Comments:

Function 03H waits for a character from standard auxiliary devices (AUX,
COM1, COM2, COM3, COM4), then returns the character in AL. This
system call does not return a status or error code.

If you press CONTROL-C, it issues Interrupt 23H.

Macro Definition:


aux_input macro
  mov ah,03H
  int 21H
  endm


Example:

The following program prints characters as soon as it receives them from
the auxiliary device. It stops printing when it receives an end-of-file char-
acter (ASCII 26, or CONTROL-Z).

begin: aux_input ;THIS FUNCTION
 cmp al,1AH ;end of file?
 je return ;yes, all done
 print_char al ;see Function 05H

  7

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

 jmp begin ;get another character



8

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Auxiliary Output (Function 04H)

 Call:

 AH = 04H
 DL
  Character for auxiliary device

 Return:

 None



Comments:

Function 04H sends the character in DL to standard auxiliary. This sys-
tem call does not return a status or error code.

If you press CONTROL-C, it issues Interrupt 23H.

Macro Definition:


aux_output macro character
  mov dl,character
  mov ah,04H
  int 21H
  endm


Example:

The following program gets a series of strings of up to 80 bytes from the
keyboard and sends each string to the auxiliary device. It stops when you
type a null string (carriage-return only).

string db 81 dup(?) ;see Function 0AH
;
begin: get_string 80,string ;see Function 0AH
 cmp string[1],0 ;null string?

  9

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_ _  |  |  _ _



_ ______________

 je return ;yes, all done
 mov cx, word ptr string[1] ;get string length
 mov bx,0 ;set index to 0
send_it: aux_output string[bx+2] ;THIS FUNCTION
 inc bx ;bump index
 loop send_it ;send another character
 jmp begin ;get another string



10

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Print Character (Function 05H)

 Call:

 AH = 05H
 DL
  Character for printer

 Return:

 None



Comments:

Function 05H sends the character in DL to the standard printer. If you
press CONTROL-C, it issues Interrupt 23H. This function does not return a
status or error code.

Macro Definition:


print_char macro character
  mov dl,character
  mov ah,05H
  int 21H
  endm


Example:

The following program prints a walking test pattern on the printer. It
stops if you press CONTROL-C.

line_num db 0
;
begin: mov cx,60 ;print 60 lines
start_line: mov bl,33 ;first printable ASCII
 ;character (!)
 add bl,line_num ;to offset one character

  11

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

 push cx ;save number-of-lines counter
 mov cx,80 ;loop counter for line
print_it: print_char bl ;THIS FUNCTION
 inc bl ;move to next ASCII character
 cmp bl,126 ;last printable ASCII
 ;character (~)
 jl no_reset ;not there yet
 mov bl,33 ;start over with (!)
no_reset: loop print_it ;print another character
 print_char 0DH ;carriage return
 print_char 0AH ;linefeed
 inc line_num ;to offset 1st char. of line
 pop cx ;restore #-of-lines counter
 loop start_line ;print another line



12

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_ _  |  |  _ _



 _ ______________


Direct Console I/O (Function 06H)

 Call:

 AH = 06H
 DL
  See text

 Return:

 AL
  If DL = FFH before call,
  then zero flag not set means AL
  has character from standard input.
  Zero flag set means there was not
  a character to get, and AL = 0.



Comments:

The action of Function 06H depends on the value in DL when the function
is called:

Value in DL
 Action
_ ________________________________________________________________

FFH If a character has been read from standard input, it is
 returned in AL and the zero flag is cleared (0); if a
 character has not been read, the zero flag is set (1).

Not FFH The character in DL is sent to standard output.

This function does not check for CONTROL-C.

Macro Definition:


dir_console_io macro switch
  mov dl,switch
  mov ah,06H
  int 21H
  endm



  13

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_ _  |  |  _ _



_ ______________


Example:

The following program sets the system clock to 0 and displays the time
continuously. When you type any character, the display freezes; when you
type any character again, the clock is reset to 0 and the display starts
again.

time db "00:00:00.00",0DH,0AH,"$" ;see Function 09H
; ;for explanation of $
;
begin: set_time 0,0,0,0 ;see Function 2DH
read_clock: get_time ;see Function 2CH
 CONVERT ch,time ;see end of chapter
 CONVERT cl,time[3] ;see end of chapter
 CONVERT dh,time[6] ;see end of chapter
 CONVERT dl,time[9] ;see end of chapter
 display time ;see Function 09H
 dir_console_io FFH ;THIS FUNCTION
 cmp al,0 ;character typed?
 jne stop ;yes, stop timer
 jmp read_clock ;no, keep timer
 ;running
stop: read_kbd ;see Function 08H
 jmp begin ;start over



14

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_ _  |  |  _ _



 _ ______________


Direct Console Input (Function 07H)

 Call:

 AH = 07H

 Return:

 AL
  Character from keyboard



Comments:

Function 07H waits for a character to be read from standard input, then
returns it in AL. This function does not echo the character or check for
CONTROL-C. (For a keyboard input function that echoes or checks for
CONTROL-C, see Function 01H or 08H.)

Macro Definition:


dir_console_input macro
   mov ah,07H
   int 21H
   endm


Example:

The following program prompts for a password (eight characters max-
imum) and places the characters into a string without echoing them.

password db 8 dup(?)
prompt db "Password: $" ;see Function 09H for
 ;explanation of $
begin: display prompt ;see Function 09H
 mov cx,8 ;maximum length of password
 xor bx,bx ;so BL can be used as index

  15

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_ _  |  |  _ _



_ ______________

get_pass: dir_console_input ;THIS FUNCTION
 cmp al,0DH ;was it a carriage return?
 je return ;yes, all done
 mov password[bx],al ;no, put character in string
 inc bx ;bump index
 loop get_pass ;get another character



16

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_ _  |  |  _ _



 _ ______________


Read Keyboard (Function 08H)

 Call:

 AH = 08H

 Return:

 AL
  Character from keyboard



Comments:

Function 08H waits for a character to be read from standard input, then
returns it in AL. If you press CONTROL-C, it issues Interrupt 23H. This
function does not echo the character. (For a keyboard input function that
echoes the character or checks for CONTROL-C, see Function 01H.)

Macro Definition:


read_kbd macro
  mov ah,08H
  int 21H
  endm


Example:

The following program prompts for a password (eight characters max-
imum) and places the characters into a string without echoing them.

password db 8 dup(?)
prompt db "Password: $" ;see Function 09H
 ;for explanation of $
begin: display prompt ;see Function 09H
 mov cx,8 ;maximum length of password
 xor bx,bx ;BL can be an index

  17

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

get_pass: read_kbd ;THIS FUNCTION
 cmp al,0DH ;was it a carriage return?
 je return ;yes, all done
 mov password[bx],al ;no, put char. in string
 inc bx ;bump index
 loop get_pass ;get another character



18

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_ _  |  |  _ _



 _ ______________


Display String (Function 09H)

 Call:

 AH = 09H
 DS:DX
  Pointer to string to be displayed

 Return:

 None



Comments:

Function 09H sends to standard output a string that ends with "$" (the $
is not displayed).

The DX register must contain the offset (from the segment address in DS)
of the string.

Macro Definition:


display macro string
  mov dx,offset string
  mov ah,09H
  int 21H
  endm


Example:

The following program displays the hexadecimal code of the key that is
typed.

table db "0123456789ABCDEF"
result db " - 00H",0DH,0AH,"$" ;see text for
 ;explanation of $
begin: read_kbd_and_echo ;see Function 01H

  19

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

 xor ah,ah ;clear upper byte
 convert ax,16,result[3] ;see end of chapter
 display result ;THIS FUNCTION
 jmp begin ;do it again



20

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Buffered Keyboard Input (Function 0AH)

 Call:

 AH = 0AH
 DS:DX
  Pointer to input buffer

 Return:

 None



Comments:

Function 0AH gets a string from standard input. DX must contain the
offset (from the segment address in DS) of an input buffer of the following
form:

Byte
 Contents
_ ________________________________________________________________

1 Maximum number of characters in buffer, including the carriage
 return (you must set this value).

2 Actual number of characters typed, not counting the carriage
 return (the function sets this value).

3-n Buffer; must be at least as long as the number in byte 1.

Characters are read from standard input and placed in the buffer begin-
ning at the third byte until a RETURN character (ASCII 0DH) is read. If the
buffer fills to one less than the maximum, additional characters read are
ignored and ASCII 07H (Bel) is sent to standard output until a RETURN
character is read. If you type the string at the console, it can be edited as
it is being entered. If you press CONTROL-C, it issues Interrupt 23H.

MS-DOS sets the second byte of the buffer to the number of characters
read (not counting the carriage return).


  21

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_ _  |  |  _ _



_ ______________


Macro Definition:


get_string macro limit,string
  mov dx,offset string
  mov string,limit
  mov ah,0AH
  int 21H
  endm


Example:

The following program gets a 16-byte (maximum) string from the key-
board and fills a 24-line by 80-character screen with it.

buffer label byte
max_length db ? ;maximum length
chars_entered db ? ;number of chars.
string db 17 dup (?) ;16 chars + CR
strings_per_line dw 0 ;how many strings
 ;fit on line
crlf db 0DH,0AH
;
begin: get_string 17,buffer ;THIS FUNCTION
 xor bx,bx ;so byte can be
 ;used as index
 mov bl,chars_entered ;get string length
 mov buffer[bx+2],"$" ;see Function 09H
 mov al,50H ;columns per line
 cbw
 div chars_entered ;times string fits
 ;on line
 xor ah,ah ;clear remainder
 mov strings_per_line,ax ;save col. counter
 mov cx,24 ;row counter
display_screen: push cx ;save it
 mov cx,strings_per_line ;get col. counter
display_line: display string ;see Function 09H
 loop display_line
 display crlf ;see Function 09H
 pop cx ;get line counter
 loop display_screen ;display 1 more line



22

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Check Keyboard Status (Function 0BH)

 Call:

 AH = 0BH

 Return:

 AL
  00H = no characters in type-ahead buffer
  FFH = characters in type-ahead buffer



Comments:

Function 0BH checks whether characters are available from standard
input (if standard input has not been redirected, it checks the type-ahead
buffer). If characters are available, AL returns FFH; if not, AL returns 0.
If CONTROL-C is in the buffer, it issues Interrupt 23H.

Macro Definition:


check_kbd_status macro
   mov ah,0BH
   int 21H
   endm


Example:

The following program displays the time continuously until you press any
key:

time db "00:00:00.00",0DH,0AH,"$"
 .
 .
begin: get_time ;see Function 2CH
 byte_to_dec ch,time ;see end of chapter
 byte_to_dec cl,time[3] ;see end of chapter

  23

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________

 byte_to_dec dh,time[6] ;see end of chapter
 byte_to_dec dl,time[9] ;see end of chapter
 display time ;see Function 09H
 check_kbd_status ;THIS FUNCTION
 cmp al,0FFH ;has a key been typed?
 je return ;yes, go home
 jmp begin ;no, keep displaying
 ;time



24

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Flush Buffer, Read Keyboard (Function 0CH)

 Call:

 AH = 0CH
 AL
  1, 6, 7, 8, or 0AH = the
  corresponding function
  is called.
  Any other value = no
  further processing.

 Return:

 AL
  00H = Type-ahead buffer was
  flushed; no other
  processing performed.



Comments:

Function 0CH empties the standard input buffer (if standard input has not
been redirected, Function 0CH empties the type-ahead buffer). Further
processing depends on the value in AL when the function is called.

AL
 Action
_ ________________________________________________________________

1,6,7,8, or 0AH The corresponding MS-DOS function is executed.

Any other value No further processing; AL returns 0.


Macro Definition:


flush_and_read_kbd macro switch
   mov al,switch
   mov ah,0CH
   int 21H
   endm



  25

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________


Example:

The following program both displays and prints characters as you type
them. If you press the RETURN key, the program sends a carriage-
return/linefeed sequence to both the display and the printer.

begin: flush_and_read_kbd 1 ;THIS FUNCTION
 print_char al ;see Function 05H
 cmp al,0DH ;is it a carriage return?
 jne begin ;no, print it
 print_char 0AH ;see Function 05H
 display_char 0AH ;see Function 02H
 jmp begin ;get another character



26

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Reset Disk (Function 0DH)

 Call:

 AH = 0DH

 Return:

 None



Comments:

Function 0DH flushes all file buffers to ensure that the internal buffer
cache matches the disks in the drives. It writes out buffers that have been
modified, and marks all buffers in the internal cache as free. This function
request is normally used to force a known state of the system; CONTROL-C
interrupt handlers should call this function.

This function does not update directory entries; you must close changed
files to update their directory entries (see Function 10H, Close File).

Macro Definition:


reset_disk macro
  mov ah,0DH
  int 21H
  endm



  27

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________


Example:

The following program flushes all file buffers and selects disk A.

begin: reset_disk
 select_disk "A"



28

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Select Disk (Function 0EH)

 Call:

 AH = 0EH
 DL
  Logical drive number
  (0 = A, 1 = B, etc.)

 Return:

 AL
  Number of logical drives



Comments:

Function 0EH selects the drive specified in DL (0=A, 1=B, etc.) as the
current logical drive. AL returns the number of logical drives.

_ ________________________________________________________________

Note

 For future compatibility, treat the value returned in AL with care.
 For example, if AL returns 5, it is not safe to assume that drives A, B,
 C, D, and E are all valid drive designators.

_ ________________________________________________________________


Macro Definition:


select_disk macro disk
  mov dl,disk[-64]
  mov ah,0EH
  int 21H
  endm



  29

_ _  |  |  _ _




_ _  |  |  _ _



_ ______________


Example:

The following program toggles between drive A and drive B to select the
current drive (in a two-drive system).

begin: current_disk ;see Function 19H
 cmp al,00H ;drive A: selected?
 je select_b ;yes, select B
 select_disk "A" ;THIS FUNCTION
 jmp return
select_b: select_disk "B" ;THIS FUNCTION



30

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Open File (Function 0FH)

 Call:

 AH = 0FH
 DS:DX
  Pointer to unopened FCB

 Return:

 AL
  00H = Directory entry found
  FFH = No directory entry found



Comments:

Function 0FH opens a file. DX must contain the offset (from the segment
address in DS) of an unopened File Control Block (FCB). This call
searches the disk directory for the named file.

If the call finds a directory entry for the file, AL returns 0 and the FCB is
filled as follows:

 o If the drive code was 0 (current drive), it is changed to the actual
 drive used (1=A, 2=B, etc.). This lets you change the current
 drive without interfering with subsequent operations on this file.

 o Current Block (offset 0CH) is set to 0.

 o Record Size (offset 0EH) is set to the system default of 128.

 o File Size (offset 0FH), Date of Last Write (offset 13H), and Time of
 Last Write (offset 15H) are set from the directory entry.

Before performing a sequential disk operation on the file, you must set the
Current Record field (offset 1FH). Before performing a random disk opera-
tion on the file, you must set the Relative Record field (offset 20H). If the
default record size (128 bytes) is not correct, set it to the correct length.

If the call doesn't find a directory entry for the file, or if the file has the
hidden or system attribute, AL returns 0FFH.


  31

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_ _  |  |  _ _



_ ______________


Macro Definition:


open macro fcb
 mov dx,offset fcb
 mov ah,0FH
 int 21H
 endm


Example:

The following program prints a file named textfile.asc that is on the disk in
drive B. If a partial record is in the buffer at end-of-file, the routine that
prints the partial record prints characters until it encounters an end-of-file
mark (ASCII 26, or CONTROL-Z).

fcb db 2,"TEXTFILEASC"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 open fcb ;THIS FUNCTION
read_line: read_seq fcb ;see Function 14H
 cmp al,02H ;end of file?
 je all_done ;yes, go home
 cmp al,00H ;more to come?
 jg check_more ;no, check for partial
 ;record
 mov cx,80H ;yes, print the buffer
 xor si,si ;set index to 0
print_it: print_char buffer[si] ;see Function 05H
 inc si ;bump index
 loop print_it ;print next character
 jmp read_line ;read another record
check_more: cmp al,03H ;part. record to print?
 jne all_done ;no
 mov cx,80H ;yes, print it
 xor si,si ;set index to 0
find_eof: cmp buffer[si],26 ;end-of-file mark?
 je all_done ;yes
 print_char buffer[si] ;see Function 05H
 inc si ;bump index to next
 ;character
 loop find_eof
all_done: close fcb ;see Function 10H



32

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Close File (Function 10H)

 Call:

 AH = 10H
 DS:DX
  Pointer to opened FCB

 Return:

 AL
  00H = Directory entry found
  FFH = No directory entry found



Comments:

Function 10H closes a file. DX must contain the offset (to the segment
address in DS) of an opened FCB. This call searches the disk directory for
the file named in the FCB. If it finds a directory entry for the file, it com-
pares the location of the file with the corresponding entries in the FCB.
The call then updates the directory entry, if necessary, to match the FCB,
and AL returns 0.

After you change a file, you must call this function to update the directory
entry. You should close any FCB (even one for a file that has not been
changed) when you no longer need access to a file.

If this call doesn't find a directory entry for the file, AL returns FFH.

Macro Definition:


close macro fcb
 mov dx,offset fcb
 mov ah,10H
 int 21H
 endm



  33

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_ _  |  |  _ _



_ ______________


Example:

The following program checks the first byte of the file named mod1.bas in
drive B to see if it is FFH and, if it is, prints a message.

message db "Not saved in ASCII format",0DH,0AH,"$"
fcb db 2,"MOD1 BAS"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 open fcb ;see Function 0FH
 read_seq fcb ;see Function 14H

 cmp buffer,0FFH ;is first byte FFH?
 jne all_done ;no
 display message ;see Function 09H
all_done: close fcb ;THIS FUNCTION



34

_ _  |  |  _ _




_ _  |  |  _ _



 _ ______________


Search for First Entry (Function 11H)

 Call:

 AH = 11H
 DS:DX
  Pointer to unopened FCB

 Return:

  AL
  00H = Directory entry found
  FFH = No directory entry found



Comments:

Function 11H searches the disk directory for the first matching filename.
DX must contain the offset (from the segment address in DS) of an
unopened FCB. The filename in the FCB can include wildcard characters.
To search for hidden or system files, DX must point to the first byte of an
extended FCB prefix.

If this call does not find a directory entry for the filename in the FCB, AL
returns FFH.

But if the call does find a directory entry for the filename in the FCB, AL
returns 0 and the call creates an unopened FCB of the same type (normal
or extended) at the Disk Transfer Address as follows:

 1. If the search FCB was normal, the first byte at the Disk Transfer
 Address is set to the drive number used in the search (1=A, 2=B,
 etc.) and the next 32 bytes contain the directory entry.

 2. If the search FCB was extended, the first byte at the Disk Transfer
 Address is set to FFH, the next 5 bytes are set to 00H, and the fol-
 lowing byte is set to the value of the attribute byte in the search
 FCB. The remaining 33 bytes are the same as the result of the nor-
 mal FCB (drive number and 32 bytes of directory entry).

If you use Function 12H (Search for Next Entry) to continue searching for
matching filenames, you must not alter or open the original FCB at
DS:DX.

  35

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_ _  |  |  _ _



_ ______________

The attribute field is the last byte of the extended FCB fields that precede
the FCB (see earlier in this chapter). If the attribute field is zero, Function
11H searches only normal file entries. It does not search directory entries
for hidden files, system files, volume label, and subdirectories.

If the attribute field is hidden file, system file, or subdirectory entry (02H,
04H, or 10H), or any combination of those values, this call also searches all
normal file entries. To search all directory entries except the volume label,
set the attribute byte to 16H (hidden file and system file and directory
entry).

If the attribute field is Volume ID (08H), the call searches only the volume
label entry.

Macro Definition:


search_first macro fcb
  mov dx,offset fcb
  mov ah,11H
  int 21H
  endm


Example:

The following program verifies the existence of a file named report.asm on
the disk in drive B.

yes db "FILE EXISTS.$"
no db "FILE DOES NOT EXIST.$"
crlf db 0DH,0AH,"$"
fcb db 2,"REPORT *ASM"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 search_first fcb ;THIS FUNCTION
 cmp al,0FFH ;directory entry found?
 je not_there ;no
 display yes ;see Function 09H
 jmp continue
not_there: display no ;see Function 09H
continue: display crlf ;see Function 09H



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 _ ______________


Search for Next Entry (Function 12H)

 Call:

 AH = 12H
 DS:DX
  Pointer to unopened FCB

 Return:

 AL
  00H = Directory entry found
  FFH = No directory entry found



Comments:

After you use Function 11H (Search for First Entry), you can use Function
12H to find any additional directory entries that match a filename (con-
taining wildcard characters). Function 12H searches the disk directory for
the next matching name. DX must contain the offset (from the segment
address in DS) of an FCB specified in a previous call to Function 11H. To
search for hidden or system files, DX must point to the first byte of an
extended FCB prefix\(emone that includes the appropriate attribute value.

If the call does not find a directory entry for the filename in the FCB, AL
returns FFH.

But if the call does find a directory entry for the filename in the FCB, AL
returns 0 and the call creates an unopened FCB of the same type (normal
or extended) at the Disk Transfer Address (see Function 11H for a descrip-
tion of how the unopened FCB is formed).

Macro Definition:


search_next macro fcb
  mov dx,offset fcb
  mov ah,12H
  int 21H
  endm


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_ ______________


Example:

The following program displays the number of files on the disk in drive B.

message db "No files",0DH,0AH,"$"
files db 0
fcb db 2,"???????????"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 search_first fcb ;see Function 11H
 cmp al,0FFH ;directory entry found?
 je all_done ;no, no files on disk
 inc files ;yes, increment file
 ;counter
search_dir: search_next fcb ;THIS FUNCTION
 cmp al,0FFH ;directory entry found?
 je done ;no
 inc files ;yes, increment file
 ;counter
 jmp search_dir ;check again
done: convert files,10,message ;see end of chapter
all_done: display message ;see Function 09H



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 _ ______________


Delete File (Function 13H)

 Call:

 AH = 13H
 DS:DX
  Pointer to unopened FCB

 Return:

  AL
  00H = Directory entry found
  FFH = No directory entry found



Comments:

Function 13H deletes a file. DX must contain the offset (from the segment
address in DS) of an unopened FCB. This call searches the directory for a
matching filename. The filename in the FCB can contain wildcard charac-
ters.

If the call does not find a matching directory entry, AL returns FFH.

But if the call does find a matching directory entry, AL returns 0 and the
call deletes the entry from the directory. If the filename contains a wild-
card character, the call will delete all files which match.

Do not delete open files.

Macro Definition:


delete macro fcb
 mov dx,offset fcb
 mov ah,13H
 int 21H
 endm



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_ ______________


Example:

The following program deletes each file on the disk in drive B that was last
written before December 31, 1982.

year dw 1982
month db 12
day db 31
files db 0
message db "No files deleted.",0DH,0AH,"$"
fcb db 2,"???????????"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 search_first fcb ;see Function 11H
 cmp al,0FFH ;directory entry found?
 jne compare ;yes
 jmp all_done ;no, no files on disk
compare: convert_date buffer ;see end of chapter
 cmp cx,year ;next several lines
 jg next ;check date in directory
 cmp dl,month ;entry against date
 jg next ;above & check next file
 cmp dh,day ;if date in directory
 jge next ;entry isn't earlier.
 delete buffer ;THIS FUNCTION
 inc files ;bump deleted-files
 ;counter
next: search_next fcb ;see Function 12H
 cmp al,00H ;directory entry found?
 je compare ;yes, check date
 cmp files,0 ;any files deleted?
 je all_done ;no, display No files
 ;message.
 convert files,10,message ;see end of chapter
all_done: display message ;see Function 09H



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 _ ______________


Sequential Read (Function 14H)

 Call:

 AH = 14H
 DS:DX
  Pointer to opened FCB

 Return:

 AL
  00H = Read completed successfully
  01H = EOF
  02H = DTA too small
  03H = EOF, partial record



Comments:

Function 14H reads a record from a specified file. DX must contain the
offset (from the segment address in DS) of an opened FCB. This call loads
the record pointed to by the Current Block field (offset 0CH) and Current
Record (offset 1FH) field at the Disk Transfer Address, then increments the
Current Block and Current Record fields.

The length of the record is taken from the Record Size field (offset 0EH) of
the FCB.

AL returns a code that describes the processing:

Code
 Meaning
_ ________________________________________________________________

0 Read completed successfully

1 End-of-file; no data in the record

2 Not enough room at the Disk Transfer Address to read one record;
 read canceled

3 End-of-file; a partial record was read and padded to the record
 length with zeros



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_ ______________


Macro Definition:


read_seq macro fcb
  mov dx,offset fcb
  mov ah,14H
  int 21H
  endm


Example:

The following program displays a file named textfile.asc that is on the disk
in drive B; its function is similar to the MS-DOS type command. If a par-
tial record is in the buffer at end-of-file, the routine that displays the par-
tial record displays characters until it encounters an end-of-file mark (ASCII
26, or CONTROL-Z).

fcb db 2,"TEXTFILEASC"
 db 26 dup (?)
buffer db 128 dup (?),"$"
;
begin: set_dta buffer ;see Function 1AH
 open fcb ;see Function 0FH
read_line: read_seq fcb ;THIS FUNCTION
 cmp al,02H ;DTA too small?
 je all_done ;yes
 cmp al,00H ;end-of-file?
 jg check_more ;yes
 display buffer ;see Function 09H
 jmp read_line ;get another record
check_more: cmp al,03H ;partial record in buffer?
 jne all_done ;no, go home
 xor si,si ;set index to 0
find_eof: cmp buffer[si],26 ;is character EOF?
 je all_done ;yes, no more to display
 display_char buffer[si] ;see Function 02H
 inc si ;bump index
 jmp find_eof ;check next character
all_done: close fcb ;see Function 10H



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 _ ______________


Sequential Write (Function 15H)

 Call:

 AH = 15H
 DS:DX
  Pointer to opened FCB

 Return:

 AL
  00H = Write completed successfully
  01H = Disk full
  02H = DTA too small


Function 15H writes a record to a specified file. DX must contain the
offset (from the segment address in DS) of an opened FCB. This call
writes the record pointed to by the Current Block field (offset 0CH) and
Current Record field (offset 1FH) at the Disk Transfer Address, then incre-
ments the Current Block and Current Record fields.

The record size is taken from the value of the Record Size field (offset 0EH)
of the FCB. If the record size is less than a sector, the call writes the data
at the Disk Transfer Address to an MS-DOS buffer; MS-DOS writes the
buffer to disk when it contains a full sector of data, when the file is closed,
or when Function 0DH (Reset Disk) is issued.

AL returns a code that describes the processing:

Code
 Meaning
_ ________________________________________________________________

0 Write completed successfully

1 Disk full; write canceled

2 Not enough room at the Disk Transfer Address to write one record;
 write canceled



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_ ______________


Macro Definition:


write_seq macro fcb
  mov dx,offset fcb
  mov ah,15H
  int 21H
  endm


Example:

The following program creates a file named dir.tmp on the disk in drive B,
containing the disk number (0=A, 1=B, etc.) and filename from each
directory entry on the disk.

record_size equ 0EH ;offset of Record Size
; field in FCB
fcb1 db 2,"DIR TMP"
 db 26 dup (?)
fcb2 db 2,"???????????"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 search_first fcb2 ;see Function 11H
 cmp al,0FFH ;directory entry found?
 je all_done ;no, no files on disk
 create fcb1 ;see Function 16H
 mov fcb1[record_size],12
 ;set record size to 12
write_it: write_seq fcb1 ;THIS FUNCTION
 cmp al,0 ;write successful?
 jne all_done ;no, go home
 search_next fcb2 ;see Function 12H
 cmp al,FFH ;directory entry found?
 je all_done ;no, go home
 jmp write_it ;yes, write the record
all_done: close fcb1 ;see Function 10H



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 _ ______________


Create File (Function 16H)

 Call:

 AH = 16H
 DS:DX
  Pointer to unopened FCB

 Return:

 AL
  00H = Empty directory found
  FFH = No empty directory available


Function 16H creates a file. DX must contain the offset (from the segment
address in DS) of an unopened FCB. MS-DOS searches the directory for
an entry that matches the specified filename or, if there is no matching
entry, an empty entry.

If MS-DOS finds a matching entry, it opens the file and sets the length to
zero (in other words, if you try to create a file that already exists, MS-DOS
erases it and creates a new, empty file). If MS-DOS doesn't find a matching
entry but does find an empty directory entry, it opens the file and sets its
length to zero. In either case, the call creates the file, and AL returns 0. If
MS-DOS doesn't find a matching entry and there is no empty entry, the
call doesn't create the file, and AL returns FFH.

You can assign an attribute to the file by using an extended FCB with the
attribute byte set to the appropriate value (see Extended FCB in Section
1.9.1).

Macro Definition:


create macro fcb
 mov dx,offset fcb
 mov ah,16H
 int 21H
 endm



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_ ______________


Example:

The following program creates a file named dir.tmp on the disk in drive B,
containing the disk number (0 = A, 1 = B, etc.) and filename from each
directory entry on the disk.

record_size equ 0EH ;offset of Record Size
; field of FCB
fcb1 db 2,"DIR TMP"
 db 26 dup (?)
fcb2 db 2,"???????????"
 db 26 dup (?)
buffer db 128 dup (?)
;
begin: set_dta buffer ;see Function 1AH
 search_first fcb2 ;see Function 11H
 cmp al,0FFH ;directory entry found?
 je all_done ;no, no files on disk
 create fcb1 ;THIS FUNCTION
 mov fcb1[record_size],12
 ;set record size to 12
write_it: write_seq fcb1 ;see Function 15H
 cmp al,0 ;write successful
 jne all_done ;no, go home
 search_next fcb2 ;see Function 12H
 cmp al,FFH ;directory entry found?
 je all_done ;no, go home
 jmp write_it ;yes, write the record
all_done: close fcb1 ;see Function 10H



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 _ ______________


Rename File (Function 17H)

 Call:

 AH = 17H
 DS:DX
  Pointer to modified FCB

 Return:

 AL
  00H = Directory entry found
  FFH = No directory entry found 
   or destination already exists


Function 17H changes the name of an existing file. DX must contain the
offset (from the segment address in DS) of an FCB with the drive number
and filename filled in, followed by a second filename at offset 11H. DOS
searches the disk directory for an entry that matches the first filename.
This filename can contain wildcard characters.

If MS-DOS finds a matching directory entry and there is no directory entry
that matches the second filename, it changes the filename in the directory
entry to match the second filename in the modified FCB. AL then returns
zero. If the second filename does contain a wildcard character, this call
does not change the corresponding characters in the filename of the direc-
tory entry.

You cannot use this function request to rename a hidden file, a system file,
or a subdirectory. If MS-DOS does not find a matching directory entry or
if it finds an entry for the second filename, AL returns FFH.

Macro Definition:


rename macro fcb,newname
 mov dx,offset fcb
 mov ah,17H
 int 21H
 endm



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_ ______________


Example:

The following program prompts for the name of a file and a new name; it
then renames the file.

fcb db 37 dup (?)
prompt1 db "Filename: $"
prompt2 db "New name: $"
reply db 15 dup(?)
crlf db 0DH,0AH,"$"
;
begin: display prompt1 ;see Function 09H
 get_string 15,reply ;see Function 0AH
 display crlf ;see Function 09H
 parse reply[2],fcb ;see Function 29H
 display prompt2 ;see Function 09H
 get_string 15,reply ;see Function 0AH
 display crlf ;see Function 09H
 parse reply[2],fcb[16]
 ;see Function 29H
 rename fcb ;THIS FUNCTION



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 _ ______________


Get Current Disk (Function 19H)

 Call:

 AH = 19H

 Return:

 AL
  Currently selected drive
  (0 = A, 1 = B, etc.)



Comments:

Function 19H returns the current drive in AL (0=A, 1=B, etc.).

Macro Definition:


current_disk macro
  mov ah,19H
  int 21H
  endm


Example:

The following program displays the default drive in a two-drive system.

message db "Current disk is $"
crlf db 0DH,OAH,"$"
;
begin: display message ;see Function 09H
 current_disk ;THIS FUNCTION
 cmp al,00H ;is it disk A?
 jne disk_b ;no, it's disk B:
 display_char "A" ;see Function 02H
 jmp all_done
disk_b: display_char "B" ;see Function 02H
all_done: display crlf ;see Function 09H

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_ ______________


Set Disk Transfer Address (Function 1AH)

 Call:

 AH = 1AH
 DS:DX
  Disk Transfer Address

 Return:

 None



Comments:

Function 1AH sets the Disk Transfer Address. DX must contain the offset
(from the segment address in DS) of the Disk Transfer Address. Disk
transfers cannot wrap around from the end of the segment to the begin-
ning, nor can they overflow into another segment.

If you do not set the Disk Transfer Address, MS-DOS defaults to offset
80H in the Program Segment Prefix. You can check the current Disk
Transfer Address with Function 2FH (Get Disk Transfer Address).

Macro Definition:


set_dta macro buffer
  mov dx,offset buffer
  mov ah,1AH
  int 21H
  endm



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 _ ______________


Example:

The following program prompts for a letter, converts it to its alphabetic
sequence (A=1, B=2, etc.), then reads and displays the corresponding
record from a file named alphabet.dat that is on the disk in drive B. The
file contains 26 records, each 28 bytes long.

record_size equ 0EH ;offset of Record Size
 ;field of FCB
relative_record equ 21H ;offset of Relative Record
; field of FCB
fcb db 2,"ALPHABETDAT"
 db 26 dup (?)
buffer db 28 dup(?),"$"
prompt db "Enter letter: $"
crlf db 0DH,0AH,"$"
;
begin: set_dta buffer ;THIS FUNCTION
 open fcb ;see Function 0FH
 mov fcb[record_size],28 ;set record size
get_char: display prompt ;see Function 09H
 read_kbd_and_echo ;see Function 01H
 cmp al,0DH ;just a CR?
 je all_done ;yes, go home
 sub al,41H ;convert ASCII
 ;code to record #
 mov fcb[relative_record],al
 ;set relative record
 display crlf ;see Function 09H
 read_ran fcb ;see Function 21H
 display buffer ;see Function 09H
 display crlf ;see Function 09H
 jmp get_char ;get another character
all_done: close fcb ;see Function 10H



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_ ______________


Get Default Drive Data (Function 1BH)

 Call:

 AH = 1BH

 Return:

 AL
  Sectors per cluster
 CX
  Bytes per sector
 DX
  Clusters per drive
 DS:BX
  Pointer to FAT ID byte


Function 1BH retrieves data about the disk in the default drive. The data
returns in the following registers:

Register
 Contents
_ ________________________________________________________________

AL Number of sectors in a cluster (allocation unit)

CX Number of bytes in a sector

DX Number of clusters on the disk

BX returns the offset (to the segment address in DS) of the first byte of the
File Allocation Table (FAT), which identifies the type of disk in the drive:

Value
 Type of Drive
_ ________________________________________________________________

FF Double-sided disk, 8 sectors per track, 40 tracks per side

FE Single-sided disk, 8 sectors per track, 40 tracks per side

FD Double-sided disk, 9 sectors per track, 40 tracks per side

FC Single-sided disk, 9 sectors per track, 40 tracks per side

F9 Double-sided disk, 15 sectors per track, 40 tracks per side

F9 Double-sided disk, 9 sectors per track, 80 tracks per side

F8 Fixed disk

This call is similar to Function 36H (Get Disk Free Space), except that it

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 _ ______________

returns the address of the FAT ID byte in BX instead of the number of
available clusters. It is also similar to Function 1CH (Get Drive Data),
except that it returns data on the disk in the default drive instead of on
the disk in a specified drive. For a description of how MS-DOS stores data
on a disk, including a description of the File Allocation Table, see Chapter
3, "MS-DOS Technical Information."

_ ________________________________________________________________

Warning 

 The FAT ID byte is no longer adequate to identify the type of drive
 being used. See Chapter 2, "MS-DOS Device Drivers," for more
 details.

_ ________________________________________________________________


Macro Definition:


def_drive_data macro
  push ds
  mov ah,1BH
  int 21H
  mov al,byte ptr[bx]
  pop ds
  endm


Example:

The following program displays a message that tells whether the default
drive is a disk or a fixed disk drive.

stdout equ 1
;
msg db "Default drive is "
dskt db "disk."
fixed db "fixed."
crlf db ODH,OAH
;
begin: write_handle stdout,msg,17 ;display message
 jc write_error ;routine not shown
 def_drive_data ;THIS FUNCTION
 cmp byte ptr [bx],0F8H ;check FAT ID byte
 jne disk ;it's a disk
 write_handle stdout,fixed,6 ;see Function 40H
 jc write_error ;see Function 40H
 jmp short all_done ;clean up & go home
disk: write_handle stdout,dskt,9 ;see Function 40H
all_done: write_handle stdout,crlf,2 ;see Function 40H
 jc write_error ;routine not shown


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_ ______________


Get Drive Data (Function 1CH)

 Call:

 AH = 1CH
 DL
  Drive (0=default, 1=A, etc.)


 Return:

 AL
  0FFH if drive number is invalid;
  otherwise, sectors per cluster
 CX
  Bytes per sector
 DX
  Clusters per drive
 DS:BX
  Pointer to FAT ID byte


Comments:

Function 1CH retrieves data about the disk in the specified drive. DL must
contain the drive number (0=default, 1=A, etc.). The data returns in the
following registers:

Register
 Contents
_ ________________________________________________________________

AL Number of sectors in a cluster (allocation unit)

CX Number of bytes in a sector

DX Number of clusters on the disk

BX returns the offset (to the segment address in DS) of the first byte of the
File Allocation Table (FAT), which identifies the type of disk in the drive:

Value
 Type of Drive
_ ________________________________________________________________

FF Double-sided disk, 8 sectors per track, 40 tracks per side

FE Single-sided disk, 8 sectors per track, 40 tracks per side

FD Double-sided disk, 9 sectors per track, 40 tracks per side

FC Single-sided disk, 9 sectors per track, 40 tracks per side



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 _ ______________

F9 Double-sided disk, 15 sectors per track, 40 tracks per side

F9 Double-sided disk, 9 sectors per track, 80 tracks per side

F8 Fixed disk

If the drive number in DL is invalid, AL returns 0FFH.

_ ________________________________________________________________

Warning 

 The FAT ID byte is no longer adequate to identify the type of drive
 being used. See Chapter 2, "MS-DOS Device Drivers" for more details.

_ ________________________________________________________________

This call is similar to Function 36H (Get Disk Free Space), except that it
returns the address of the FAT ID byte in BX instead of the number of
available clusters. It is also similar to Function 1BH (Get Default Drive
Data), except that it returns data on the disk in the drive specified in DL
instead of the disk in the default drive. For a description of how MS-DOS
stores data on a disk, including a description of the File Allocation Table,
see Chapter 3, "MS-DOS Technical Information."

Macro Definition:


drive_data macro drive
  push ds
  mov dl,drive
  mov ah,1BH
  int 21H
  mov al, byte ptr[bx]
  pop ds
  endm


Example:

The following program displays a message that tells whether drive B is a
disk or a fixed disk drive.

stdout equ 1
:
msg db "Drive B is "
dskt db "disk."
fixed db "fixed."
crlf db ODH,OAH
;
begin: write_handle stdout,msg,11 ;display message
 jc write_error ;routine not shown
 drive_data 2 ;THIS FUNCTION

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_ ______________

 cmp byte ptr [bx],0F8H ;check FAT ID byte
 jne disk ;it's a disk
 write_handle stdout,fixed,6 ;see Function 40H
 jc write_error ;routine not shown
 jmp all_done ;clean up & go home
disk: write_handle stdout,dskt,9 ;see Function 40H
all_done: write_handle stdout,crlf,2 ;see Function 40H
 jc write_error ;routine not shown



56

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